1. Field of the Invention
The present invention relates to a luminescent material for a mercury discharge lamp and more particularly to a luminescent material for a fluorescent lamp, which includes a phosphor material emitting a luminous flux upon excitation by ultraviolet radiation having a wavelength of 254 nm.
2. Description of the Related Art
A mercury vapor discharge lamp, such as a fluorescent lamp is widely used as a light sources for general illumination purposes, office automation machines, light-emitting elements of the giant screen color display apparatus and back lights for liquid crystal displays. The fluorescent lamp has a luminescent layer composed of a phosphor and formed on an inner envelope of a glass tube in which mercury and one or more rare gases are filled. Electrodes are opposingly arranged in the glass tube and a discharge is caused therebetween. Mercury in the glass tube is vaporized by the discharge, and mainly ultraviolet radiation is produced. A comparatively small part of the ultraviolet radiation has a wavelength of 185 nm while the greatest part has a wavelength of 254 nm. Upon excitation of the phosphor by the ultraviolet radiation, a luminous flux or visible light is produced. The visible light is irradiated outside the glass tube. As for the general illumination purposes, a three band type fluorescent lamp, which efficiently generates white light and has excellent color rendering properties, has become popular. The luminescent layer of the three band type fluorescent lamp is composed of blue, green and red emitting phosphors each having a relatively narrow band emission spectrum distribution.
The fluorescent lamp mentioned above has a drawback because the luminous efficacy is reduced as the number of operating hours increases. In particular the reduction of the luminous efficacy in a high wall loaded fluorescent lamp is considerably higher. As disclosed in Japanese Patent Disclosure (Kokoku) 3-45505, the reduction in the luminous efficacy has been assumed to be caused by the following two factors. First, the phosphor material itself is deteriorated by ultraviolet radiation having a wavelength of 185 nm. Although the intensity ratio of ultraviolet radiation having a wavelength of 185 nm to that of a wavelength of 254 nm is about 10 to 12%, it becomes more than 12% when the fluorescent lamp is operated with a relatively high wall load as compared to the conventional fluorescent lamp.
The lamp tube can be formed in various shapes like a circle, a U-shape, a saddle-shape and other complicated shapes in addition to the ordinary straight tube. As the fluorescent lamp has been made smaller in size, the wall load of the lamp increases. A phosphor material coated on the small lamp tube is deteriorated and the luminous flux emitted from the phosphor material is lowered accordingly.
Second, the reduction of the luminous efficacy is caused by blackening of the lamp partly or fully on the inner wall of the lamp tube. The blackening is mainly due to the deposition of residues of organic binders and/or electrode substances. However, the adhesion of mercury or its compound to the phosphor material coated as the luminescent layer on the inner wall of the lamp tube is believed to cause the blackening. The reason why mercury adheres to the phosphor material is supposedly because of attractive forces exerted on mercury and the phosphor material. It is also believed that the phosphor material is deteriorated by ion and/or atom bombardments.
The Japanese Patent Disclosure (Kokoku) 3-45505 discloses phosphor materials as preferred illuminating materials for fluorescent lamps operated with a relatively high wall load. The phosphor materials are less deteriorative against ultraviolet radiation at 185 nm and less reactive to mercury. However, the phosphor materials are required to satisfy a property of having at 254 nm-excitation a luminous flux, which, after the phosphor materials have been subjected for 15 minutes to ultraviolet radiation of a wavelength of mainly 185 nm and 254 nm, a radiation density between 150 and 500 W/m.sup.2, and a ratio of 185 nm power between 0.20 and 0.40, is not more than 5% lower than the initial luminous flux of the phosphor materials also at the 254 nm excitation and measured in identical circumstances. A combination of cations in the phosphor materials has an electronegativity of not more than 1.4.